1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system.
2. Description of the Related Art
A type of the sheet processing apparatus that performs folding processing on sheets including images formed by an image forming apparatus has been known. A sheet processing apparatus described in Japanese Laid-open Patent Publication No. 2003-341930 includes a folding plate that pushes a sheet surface of a sheet bundle having a plurality of sheets bundled at a folding position in a direction orthogonal to the sheet surface using an end of the folding plate, and a pair of folding rollers (hereinafter, “folding roller pair”) that are arranged to face each other across a transfer path of the folding plate and that sandwich the sheet bundle. The sheet surface is pushed by the folding plate to a sheet folding part and then the sheet bundle is conveyed with opposite side surfaces of the sheets at the folding position sandwiched by the folding roller pair, thereby folding the sheet bundle.
Furthermore, one additional folding roller provided downstream of a sheet conveying direction with respect to the folding roller pair moves on the sheet bundle having one fold line formed thereon in a direction of the fold line while applying a pressure on the fold line of the sheet bundle, thereby additionally folding the sheet bundle.
However, the folding roller pair is configured to have a width larger than a sheet width and to hold the entire region of the sheet bundle in a sheet width direction. Additional folding processing performed by the additional folding roller is often performed near the folding roller pair because of a space in the apparatus. Accordingly, the sheet bundle is in a state being held by the folding roller pair when additional folding is performed by the additional folding roller.
A fold line portion of the sheet bundle entering a nip part of the additional folding roller only slightly protrudes from a nip part of the folding roller pair. The sheet bundle having passed through the nip part of the folding roller pair tends to expand at the fold line due to firmness of the sheets. However, because a distance between the fold line of the sheet bundle and the folding roller pair is short and the sheets are held by the folding roller pair, the fold line of the sheet bundle hardly expands. Therefore, there are almost no gaps between adjacent sheets, respectively, at the fold line of the sheet bundle and the sheets are closely superposed one on top of another.
Basically, folding of a sheet is achieved by deforming the sheet and breaking fibers of the sheet to form a crease on the sheet. Therefore, when the sheets are closely superposed one on top of another at the fold line of the sheet bundle, spaces for deforming the sheets are not provided between adjacent sheets, respectively, at the fold line. Accordingly, the sheets are hardly deformed even when additional folding is performed, so that a firm crease cannot be formed at the fold line. Furthermore, because the single additional folding roller applies a pressure on the fold line of the sheet bundle, there is still one fold line in the sheet bundle after the additional folding, which prevents a folded height of the sheet bundle from being sufficiently reduced.
Therefore, there is a need to provide a sheet processing apparatus that can reduce a folded height of a sheet bundle and an image forming system including the sheet processing apparatus.